1. Field of the Invention
This invention relates to an improvement in modular plugs for terminating a cord having a nonplanar array of conductors enclosed in a plastic jacket, that is, a jacket of substantially circular, or oval, peripheral shape. More particularly, the invention relates to improvements in plugs that are especially suited for terminating retractile telephone cords, especially round cords, to facilitate connection of such cords to modular jacks for use in modular telephone systems.
2. State-of-the-Art
For some time now, components of telephone systems have been marketed on a modular basis, so that telephone users may select various components at stores known as Phone-Center stores, or Phone Marts, and the suppliers of telephone systems, that is, the telephone companies themselves, have encouraged this by themselves providing modular components. More particularly, the telephone base that includes the dialing and ringing apparatus and the handset are supplied separately on a modular basis, and both the cord for connecting the telephone base to a wall terminal and the cord for interconnecting the telephone base with the telephone handset, are separately supplied and sold. The cords are supplied with modular plugs at each end which mate with jacks, or sockets, that are installed in the wall terminal, in the telephone base, and in the handset.
The cords themselves typically include a plurality of individually-insulated electrical conductors and a plastic insulating jacket in which they are enclosed. The cord that connects the telephone handset with the telephone base is generally of the retractile-type, such as the three-conductor round cord that is disclosed in Cox U.S. Pat. No. 2,609,417. In that round cord, as in many other round cords, the conductors are bunched together snugly. In four-conductor round cords commonly in use, the round cord is of oval configuration with major and minor axis of about 0.200 inch and 0.170 inch respectively, but in the coiled form are normally slightly flattened on the interior side of the coil.
More modernly, the conductors are arranged in a common plane in what is called a flat cord, such as described in Hardesty U.S. Pat. No. 4,002,392. But the terminals of jacks and plugs are often arranged in multiple planes or tiers. New installations generally involve such flat cords and such jacks and plugs. But where modular equipment is being supplied to replace old non-modular telephone apparatus, attempts are made to salvage the old round cords. This is done by cutting the ends and mounting modular plugs on them which are adapted to mate with modern modular jacks that are supplied for use with flat cords. Since there are over 100,000,000 non-modular telephones with round cords, the salvaging of these cords for use in modern modular telephone systems represents an important way to conserve natural resources, and the dollars of telephone users.
Modular plugs of the type that have been employed with flat cords are disclosed, for example, in Hardesty et al U.S. Pat. No. 3,761,869, Hardesty U.S. Pat. No. 3,860,316, Hardesty U.S. Pat. No. 3,998,514, and Hardesty U.S. Pat. No. 4,002,392. In all of those patents, a plug is provided that has a housing formed with a cavity in which a flat cord is enclosed with the conductors extending into cells where the conductors are electrically contacted by terminals. These terminals are arranged to make electrical connection with mating terminals of a jack.
In these plugs, an anchoring member pivotally mounted in a laterally extending side wall of the housing and pivoted to it, is pressed into anchoring engagement with the cord jacket. Attempts have been made to employ the same type of plug to clamp round cords in place in the plugs. Such an effort is represented by the Hardesty U.S. Pat. No. 4,054,350.
Experience has shown that when such plugs are used for terminating round cords, they are successful in satisfying specifications so far as longitudinal, or axial, forces are concerned. In other words, the plugs will withstand a 20-pound steady pull on the cord. But a large fraction of the round cords on which such modular plugs are mounted fail, when the cords are twisted. And such twisting often occurs during ordinary use of telephones on which such cords are used. To appreciate this problem, it will be recalled that the conductors are in the form of tinsel ribbons wound on insulating cores, such as linen or nylon thread. Cords made in this way are described, for example, in the aforementioned Cox et al patent. The terminals of the plugs are in the form of blades with points that pierce the insulating material on the individual conductors to make electrical contact with these tinsel wires or ribbons. The twisting of the conductors loosen or weaken the electrical contact between the conductors and these terminal blades. As a result, the electrical connections become subject to intermittent or permanent disconnection, rendering the telephone noisy or even completely useless.
It is an object of this invention to provide an improved modular plug especially suited for terminating cords that have generally circular configuration.
A further object of the invention is to provide an improved modular plug that is adapted to mate with jacks that are also adapted to mate with modular plugs used on flat cords.
It is a further object of the invention to provide a modular plug with improved means for resisting rotation and twisting of a round cord on which it is mounted.
It is a further object of the invention to provide an improved modular plug which can be more firmly secured to a telephone cord that it terminates.